Method for mounting a semiconductor package onto PCB

ABSTRACT

A method for mounting a semiconductor package onto PCB is disclosed. A semiconductor package is provided, which comprises a plurality of outer terminals exposed out of an encapsulant. A PCB is provided and has a surface with a plurality of contact pads. Each contact pad has a first exposed side, an opposing exposed second side and a center between the exposed sides. A plurality of first pre-solders and a plurality of second pre-solders are formed on the surface by one single printing. The first pre-solders and the second pre-solders respectively cover the first and second exposed sides of the contact pads, spaces between the opposing first and second pre-solders expose the centers of the contact pads. Then the first and second pre-solders are reflowed and the semiconductor package is mounted on the PCB. When the outer terminals is connected to the contact pads, each first pre-solder and second pre-solder are flowed toward the corresponding exposed centers of the contact pad to prevent the fine pitch contact pads from bridging.

FIELD OF THE INVENTION

The present invention relates to a method for mounting a semiconductorpackage onto PCB, particularly to a method for mounting a semiconductorpackage onto PCB with pre-solders.

BACKGROUND OF THE INVENTION

A well-known SMT technology is used for mounting a semiconductor packagewith a plurality of outer terminals on a printed circuit board (PCB).Because of the outer terminals of the semiconductor package become moreand more, the pitch of the outer terminals also becomes smaller andsmaller. Conventionally all the outer terminals have to mount on thecorresponding contact pads of the PCB via solder paste. A plurality ofpre-solders are formed on the contact pads prior to SMT processes. Whenthe outer terminals of the semiconductor package are pressed on thepre-solders, the flowing direction of the pre-solders becomesuncontrollable which may easily spread out and cause bridging betweentwo adjacent contact pads.

A method of surface mounting semiconductor package is disclosed inR.O.C. Taiwan Patent No. 483299. A semiconductor package has an exposeddie pad. A solder receiving groove is formed on the outside surface ofthe die pad. A first solder is formed on a ground panel of a printedcircuit board (PCB). A plurality of second solders are formed on aplurality of fingers of the PCB. During the reflowing process, the firstsolder connects the ground panel and die pad, the second solders connectthe fingers and the outer leads of the semiconductor package. The solderreceiving groove can receive the first solder to avoid solder beadsformation and contamination to the outer leads or fingers. However,bridging problems of the second solders between the fingers cannot besolved by means of the solder receiving groove.

SUMMARY

A main object of the present invention is to provide a method formounting a semiconductor package onto PCB. A plurality of firstpre-solders and a plurality of second pre-solders are formed on aprinted circuit board (PCB) with a plurality contact pads. The firstpre-solders cover the first exposed sides of the contact pads, and thesecond pre-solders cover the second exposed sides of the contact pads.Spaces between the first pre-solders and the corresponding secondpre-solders expose the centers of the contact pads. When the first andsecond pre-solders are reflowed and a semiconductor package is mounts onthe printed circuit board, the first pre-solders and second pre-soldersflow toward the centers of the contact pads to prevent bridging problembetween adjacent contact pads.

A second object of the present invention is to provide a method formounting a semiconductor package onto PCB. First and second pre-soldersare formed on a printed circuit board (PCB) with a plurality contactpads. The first and second pre-solders covers the two opposing sides ofthe contact pads and exposes the centers of the contact pads. When thefirst and second pre-solders are reflowed and a semiconductor package ismounted on the printed circuit board, the outer terminals of thesemiconductor package are connected to the contact pads via the firstand second pre-solders. The first and second pre-solders can extend ontothe solder mask of the PCB without bridging between the contact pads forfine pitch layout, for example, the pitch of the contact pads can beless than 0.3 mm.

According to the present invention, a method for mounting asemiconductor package onto PCB includes several important steps asfollows. A semiconductor package and a printed circuit board areprovided. The semiconductor package has an encapsulant, a chip insidethe encapsulant and a plurality of outer terminals exposed out of theencapsulant. The printed circuit board has a surface with a plurality ofcontact pads and a solder mask. Each contact pad has a first exposedside, an opposing exposed second side and a center between the first andsecond exposed sides. A plurality of first pre-solders and a pluralityof second pre-solders, and is formed on the surface of the PCB. Thefirst pre-solders cover the first exposed sides, and the secondpre-solders cover the second exposed sides. Spaces between the firstpre-solders and the corresponding second pre-solders expose the centersof the contact pads. Preferably, the first pre-solders and the secondpre-solders can extend onto the solder mask. When the first and secondpre-solders are reflowed and the semiconductor package is mounted on thesurface of the PCB, each first pre-solder and the corresponding secondpre-solder can flow toward the center of corresponding contact pad toprevent bridging problem of adjacent contact pads for the fine pitchlayout.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of a method for mounting a semiconductor packageonto PCB in accordance with the embodiment of the present invention.

FIG. 2 is a cross-sectional view of a provided semiconductor package inaccordance with the embodiment of the present invention.

FIG. 3 is a cross-sectional view of a provided printed circuit board inaccordance with the embodiment of the present invention.

FIG. 4 is a cross-sectional view of the first and second pre-soldersformed on the printed circuit board in accordance with the embodiment ofthe present invention.

FIG. 5 is an enlarged cross-sectional view of one of the contact pads onthe printed circuit board including one of outer terminals of asemiconductor package and the first and second pre-solders during SMT inaccordance with the embodiment of the present invention.

FIG. 6 is a top view of the first and second pre-solders on a printedcircuit board in accordance with the embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Referring to the drawings attached, the present invention will bedescribed by means of an embodiment below.

As shown in FIG. 1, a method for mounting a semiconductor package ontoPCB is illustrated according to the embodiment of the present invention,which mainly includes step 1 of “providing a semiconductor package”,step 2 of “providing a PCB”, step 3 of “forming first and secondpre-solders on the PCB” and step 4 of “mounting the semiconductorpackage on the PCB”.

Referring to FIG. 2, firstly in the step 1, a semiconductor package 10is provided. The semiconductor package 10 includes an encapsulant 11, achip inside the encapsulant 11 and a plurality of outer terminals 12exposed out of the encapsulant 11. The chip inside the encapsulant 11 iselectrically connected to the outer terminals 12 (not shown in thefigure). The outer terminals 12 are selected from the group consistingof gull leads, J-leads and pads of non-leaded packages. In thisembodiment, the semiconductor package 10 is a quad flat package (QFP),and the outer terminals 12 are gull leads. The semiconductor package 10has high pin count and the outer terminals 12 are arranged in finepitch. The pitch of the outer terminals 12 can be less than 0.3 mm.

Referring to FIG. 3, in the step 2, a printed circuit board (PCB) 20 isprovided. The printed circuit board 20 may be a motherboard, acommunication board or a module substrate. The printed circuit board 20has a surface 21 for surface mounting. A plurality of contact pads 22are formed on the surface 21 of the printed circuit board 20 forconnecting the outer terminals 12 of the semiconductor package 10.Preferably, a solder mask 23 is formed on the surface 21 of the printedcircuit board 20. The solder mask 23 has a plurality of openings 231 atleast exposing the contact pads 22. The openings 231 can be aligned withthe contact pads 22 to allow the contact pads 22 as SMD (Solder MaskDefined) or NSMD) (Non-Solder Mask Defined) pads. Each contact pad 22has a first exposed side 221, an opposing second exposed side 222 and acenter 223 between the first exposed side 221 and the second exposedside 222. The first exposed side 221, the second exposed sides 222 andthe centers 223 are exposed out of the solder mask 23. In thisembodiment, the first exposed sides 221 are parallel to thecorresponding second exposed sides 222. Preferably, the first exposedsides 221 and the corresponding second exposed sides 222 are the twoshorter sides of the rectangles, as shown in FIG. 6.

Referring to FIGS. 4 and 6, in step 3, a plurality of first pre-solders31 and a plurality of second pre-solders 32 are formed on the printedcircuit board 20 in a determined pattern. The first and secondpre-solders 31,32 can be formed by screen-printing or stencil-printing.The first pre-solders 31 cover the first exposed sides 221 of thecontact pads 22 and extend onto the solder mask 23. The secondpre-solders 32 cover the second exposed sides 222 of the contact pads 22and extend onto the solder mask 23. Spaces between the first pre-solders31 and the corresponding second pre-solders 32 expose the centers 223 orthe other portions of the contact pads 22. The first and secondpre-solders 31,32 corresponding to each contact pad 22 may have a shapeselected from the group consisting of “I I”, “H”, “X” and “——”. Becausethe exposed centers 223 are not covered by the first and the secondpre-solders 31 and 32, a flowing direction of the first and secondpre-solders 31,32 is provided toward the centers 223 during the mountingstep 4. Therefore, the bridging problem between adjacent contact pads 22is prevented. Besides, the step 3 of “forming first and secondpre-solders on the PCB” can be proceeded in the step 2 of “providing aPCB” as a sub-step included in the step 2, i.e., the printed circuitboard 20 with the first pre-solders 31 and the second pre-solders 32 isprovided in the step 2.

Referring to FIGS. 5 and 6, the mounting step 4 is preformed. Thesemiconductor package 10 is mounted on the surface 21 of the printedcircuit board 20. At the same time, the first and second pre-solders31,32 are reflowed. The outer terminals 12 of the semiconductor package10 are connected to the contact pads 22 via the first and secondpre-solders 31,32 in specific pattern. The first pre-solders 31 and thesecond pre-solders 32 are pressed by the outer terminals 12 then heatedso as to flow toward the corresponding centers 223 of the contact pads22. The first pre-solders 31 and the second pre-solders 32 are connectedto each other at the centers 223 on the corresponding contact pads 22during the reflowing. The first pre-solders 31 and the secondpre-solders 32 are under control and can not spread out of the soldermask 23, so there is no bridging problems between the adjacent contactpads 22. In addition, the shape of the first and second pre-solders31,32 and the locations of the exposed portions of the contact pads 22out of the first and second pre-solders 31,32 at the centers 223 are notlimited, i.e., the contact pads 22 can have various exposed portions outof the first and second pre-solders 31,32 to control the flowingdirection of the first and second pre-solders 31,32. Besides, thecontact pads 22 may be in circular or elliptic shape (not shown in thefigure), and the first pre-solders 31 and the second pre-solders 32 canbe connected together in various solder patterns. Using the first andsecond pre-solders 31,32 on the solder mask 23 to expose thepredetermined portions of the contact pads 22, the bridging problembetween adjacent contact pads 22 can be prevented when the semiconductorpackage 10 is mounted on the surface 21 of the printed circuit board 20with fine pitch contact pads 22.

The above description of embodiments of this invention is intended to beillustrated but not limited. Other embodiments of this invention will beobvious to those skilled in the art in view of the above disclosure.

1-11. (canceled)
 12. A printed circuit board having a surface with a plurality of contact pads, each contact pad having a first exposed side, an opposing exposed second side and a center between the exposed sides, the printed circuit board comprising; a plurality of first pre-solders and a plurality of second pre-solders on the surface of the printed circuit board, wherein the first pre-solders cover the first exposed sides, the second pre-solders cover the second exposed sides, the spaces are formed between the first pre-solders and the corresponding second pre-solders to expose the centers of the contact pads.
 13. The printed circuit board in according with claim 12, further comprising a solder mask on the surface of the printed circuit board, the solder mask having a plurality of openings for exposing the contact pads.
 14. The printed circuit board in according with claim 13, wherein the first and second pre-solders extend onto the solder mask.
 15. The printed circuit board in accordance with claim 12, wherein each first exposed side is parallel to the corresponding second exposed side.
 16. The printed circuit board in accordance with claim 12, wherein the first exposed sides and the corresponding second exposed sides are the two shorter sides of the rectangles.
 17. The printed circuit board in accordance with claim 12, wherein the pitch of the contact pads is less that 0.3 mm. 